A comprehensive epidemiologic study is proposed for investigating risk factors in Bronchopulmonary Dysplasia (BPD), a chronic pulmonary disease which occurs in premature neonates. In the first year, the study aims to standardize diagnosis and develop a scoring system for grading the severity of acute and chronic aspects of the disease. Subsequently, a multifactorial investigation will be undertaken to elucidate the relationship between BPD, acute lung disease severity, respiration related variables, water balance, nutrition, familial predisposition, environmental, pregnancy and delivery parameters. Important features in the study are uniform diagnostic criteria, large sample size and prospective data collection. All infants below 1,501 grams at birth in six neonatal intensive care units in Wisconsin will be entered in the study. Baseline data and standardized hospital logs will be submitted for all these infants. For neonates surviving 7 days, maternal interviews will be conducted. Severity scoring and diagnosis will be based on the hospital log and routine x-rays. Final analysis will compare infants with BPD to those without BPD, controlling for confounders and placing special emphasis on interaction effects. Additionally, a group of full term neonates will serve as an overall control.